Capacitor overcharge protection circuit

ABSTRACT

Photo flash apparatus having two storage capacitors connected in series. Normally both capacitors are charged by a voltage doubling circuit connected to an alternating current supply, to a predetermined direct current voltage across both capacitors in series. To prevent overcharging and possible breakdwon of one capacitor in case the charge on the other capacitor decreases as a result of leakage current, the invention provides a safety circuit having two resistors in series with each other and in parallel with the respective capacitors, the safety circuit having a central connection from the junction between the two resistors to the junction between the two capacitors, the central connection containing an impulse transformer which operates when the charges on the two capacitors differ by more than a predetermined amount. The output of the impulse transformer can be used to give an indication of fault or to interrupt automatically the charging of the capacitors.

United States Patent Strauss [4 Aug. 1, 1972 [54] CAPACITOR OVERCHARGE[57] ABSTRACT PROTECTION CIRCUIT Photo flash apparatus having twostorage capacitors [72] Inventor: Karl-Peter Strauss, Braunschweig,connected in series. Normally both capacitors are Germany charged by avoltage doubling circuitconnected to an [73] Assigneez Rouei werkeFranke & Heidecke alternating current supply, to a predetermined directBraunschweig, Germany [22] Filed: July 1, 1971 [21] Appl. No.: 158,957 I[52] US. Cl ..320/l, 307/110, 323/22 R [51] Int. Cl ..H03k 1/02, G05fH46 [58] Field of Search ....'.....320/1; 307/1 10; 317/12 A; 323/22 R[56] References Cited UNITED STATES PATENTS 3,417,306 12/1968 Knak..320/1 3,526,821 9/1970 Thomas ..320/l 3,634,671 1/1972 Swarbrick..320/1 X Primary Examiner-Bernard Konick I Assistant ExaminerStuartl-lecker Att0rney-Charles Shepard et a1.

current voltage across both capacitors in series. To preventovercharging and possible breakdwon of one capacitor in case the chargeon the other capacitor decreases as a result of leakage current, theinvention provides a safety circuit having two resistors in series witheach other and in parallel with the respective capacitors, the safetycircuit having a central connection from the junction between the tworesistors to the junction between the two capacitors, the centralconnection containing an impulse transformer which operates when thecharges on the two capacitors differ by more than a predeterminedamount. The outputof the impulse transformer can be used to give anindica tion of fault or to interrupt automatically the charging of thecapacitors.

9 Claims, 3 Drawing Figures I 1 V 3 14 U 15 g CAPACITOR OVERCHARGEPROTECTION CIRCUIT BACKGROUND OF THE INVENTION voltage doubling circuitsof the Greinacher type. It is advantageous to use capacitors connectedin series, because the dielectric strength of the customary electrolyticcapacitors is limited, and v the connection of two such capacitors inseries enables a higher working voltage to be obtained. It is thuspossible, by the use of voltage doubling circuits of the Greinachertype, to produce a total direct current voltage, across the twocapacitors in series, amounting to 5.65 times the effective alternatingcurrent voltage of the supply mains or circuits, with direct operationof the flash apparatus from the usual alternating current supply mains,

without recourse to. transformers.

The apparatus is usually equipped with disconnection means whichinterrupts the charging of the capacitors when the desired total directcurrent voltage is reached. However, if the charging voltage of onecapacitor decreases (e.g., as a result of leakage current) then that ofthe other capacitor increases correspondingly, since the cumulative ortotal voltage across both capacitors remains the same. The increase involtage on one capacitor may exceed the dielectric strength of thatcapacitor, so that it likewise fails.

An object of the invention is to provide a circuit which takes intoaccount the lack of symmetry of the charges on the two capacitors andwhich, whenever the difference in voltage across the two capacitorsexceeds a predetermined amount, will generate an impulse which will givea warning signal or indication to the user of the apparatus, or whichwill preferably shut off the charging current to one or both of thecapacitors.

Another object of the invention is the provision of a circuit of thekind just mentioned, which is comparatively simple, relativelyinexpensive, relatively easy'to construct, and sturdy and reliable inuse.

According to the invention, two resistors are arranged in series witheach other and in parallel to the respective capacitors. The centralconnections or junctions of the capacitors and of the resistors areconnected by a capacitor and an impulse transformer diac series circuit.When an unsymmetrical condition occurs, a voltage is produced in thecentral branch which includes the impulse transormer, the polarity ofthe voltage indicating which of the two capacitors has the lower andwhich has the higher partial voltage. If the voltage difference reachesa certain predetermined limit, then the diac ignites or fires, and thecentral branch capacitor discharges via the impulse transformer. Theoutput of the impulse transformer can be used to light a lamp to give anoptical or visual signal or indication of the fault, or the output canbe used for automatic temporary interruption of the charging of thecapacitors.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a wiring diagramillustrating the charging FIG. 2 is a wiring diagram showing oneembodiment of the safety circuit which operates as a result ofunsymmetrical charging of the two series capacitors; and

FIG. 3 is a wiring diagram of a complete circuit including provision forinterrupting the charging of the storage capacitors when there is apredetermined difference in the charges on the two capacitors.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. '1,there is shown a circuit for charging the main storage capacitors l and2, connected in series with each other, through trigger controlrectifiers 3 and 4, directly from alternating current mains connected tothe input terminals 5 and 6 by means of any conventional plug adapted tobe plugged into a conventional alternating current outlet. In a mannerwhich will be understood by those skilled in the art, the capacitor 1 ischarged to the voltage U,, and the capacitor 2 is charged to the voltageU so that the total or cumulative direct voltage U (equal to the sum ofU, and U is available across the terminals 7 and 8 and can be used forflashing a photo flash tube.

In FIG. 2, there is shown the supplementary or safety circuit accordingto one embodiment of the present invention. The capacitors l and 2 arecharged as before, through the charging circuit shown in FIG. 1. Tworesistors 9 and 10 are arranged as shown, in series with each other andin parallel with the main storage capacitors l and 2. This safetycircuit includes not only the resistors 9 and 10, but also what may becalled a central circuit or a bridge circuit, connecting the junction orcentral point 11 between the two capacitors 1 and 2 to the junction orcentral point 12 between the two resistors 9 and 10. The two capacitorsl and 2 are normally of the same capacity and are intended to be chargedto the same voltage, so that u, U In these circumstances, the resistors9 and 10 likewise have the same resistance values. However, ifunsymmetrical distribution of the voltage in the capacitors l and 2 isintended, then the resistors 9 and 10 have different values in the sameratio as the respective capacities of the capacitors 1 and 2.

The above mentioned bridge circuit or central circuit connecting thejunctions 11 and 12 has a capacitor 13, a diac l4, and the primarywinding 15 of an impulse transformer. As illustrated in FIG. 2, the diacand the transformer are in series with each other, and in parallel withthe capacitor 13. There may also be, if desired, a resistor 16 asfurther mentioned below, arranged in parallel with the capacitor 13 andin parallel with the diac and transformer combination.

If the voltage of the capacitors 1 and 2 is equal, ie if U, U then thecentral points or junction points 11 and 12 are at the same potential,and there will be no flow of current or difference in potential acrossthe bridge circuit. However, if the voltage in one capacitor (say thecapacitor 1) decreases, due to leakage currents or other fault, then theother voltage in the capacitor 2 will increase, since the sum of U, Uremains constant. In this case, a potential difference or voltage occursbetween the points 11 and 12, so that the capacitor 13 is charged up tothis voltage. When this voltage exceeds the ignition voltage or firingvoltage of the diac 14, the capacitor 13 discharges through the windingof the impulse transformer. The circuit responds when the differencebetween the separate voltages is half as great as the ignition voltageof the diac 14. The response threshold can be increased if a resistor 16is connected in parallel with the capacitor 13, as mentioned above. If alower response threshold is desired, this resistor 16 may be omitted.

If U, and U are the voltages of the charges on the capacitors l and 2,and R, and R are the values of the resistors 9 and 10, respectively, andR is the value of the resistor 16, then the voltage will be as follows:

In place of the diac 14, another element with bipolar break-through,such as a glow lamp, can be employed. If a bridge rectifier is connectedup in series, it is also possible to use an element with a unipolarbreak-through, such as a four-layer diode, or a unijunction .transistor,or the like: The system will respond if equilibrium is destroyed byshort circuit or other damage to one of the resistors 9 and 10, as wellas to lack of equilibrium in the main storage capacitors.

Referring now to FIG. 3, there is shown an embodiment of the inventionin which the impulse generated in the transformer is used forselectively shutting off the particular capacitor which is endangered.Those parts which correspond to parts already described in connectionwith FIGS. 1 and 2 have been given the same reference numerals, 'andrequire no special further description.

The trigger impulses for the rectifier thyristors 3 and 4 are introducedfrom the terminals 17 and 18, respectively. Capacitors 21 and 22 arecharged with an auxiliary voltage through the terminals 23 and 24, andare respectively connected to the trigger feed lines via the diodes l9and 20, respectively. Parallel with the capacitors 21 and 22 are thethyristors 25 and 26, respectively, which can be selectively cut throughby the ignition impulses from the secondary windings 27 and 28,respectively, of the impulse transformer 15, via the diodes 29 and 30,respectively.

Normally the capacitors 1 and 2 are charged up to the same voltage, andcapacitors 21 and 22 are charged with an auxiliary voltage taking theopposite direction. The connection of the secondary windings 27 and 28of the impulse transformer 15 in opposite direction insures inconjunction with the diodes 29 and 30 that the impulse generated whenthe diac 14 becomes conductive will be conveyed either to the thyristor25 or to the thyristor 26, as the case may be.

During the charging of the main capacitors 1 and 2, if the voltage ofthe capacitor 1 lags behind that of the capacitor 2 to a sufficientextent that the diac 14 fires and an impulse is set up in thetransformer winding 15, then the thyristor 26 is ignited. The associatedcapacitor 22 is thus discharged. By this process the trigger impulsesare once again short circuited via the diode and the dischargedcapacitor 22, until the capacitor 22 has once again been charged up. Aslong as this recharging proceeds, the continued charging of the storagecapacitor 2 is interrupted. It is alsopossible for the interruption ofthe charging to be effected for both storage capacitors at the sametime, if desired.

This control system is also applicable to cascade circuits of 4, 6, ormore storage capacitors, by building up a number of bridge circuits inaccordance with the teaching of FIG. 2. In this case, each storagecapacitor can be selectively shut;, off by using circuits in accordancewith FIG. 3. It is also possible for the entire cascade system to beshut off permanently or temporarily in the event of faults in one of thestorage capacitors or one of the associated resistors.

Assuming that the charging of the storage capacitors proceeds normally,so that no operation of the safety system or protection system of thepresent invention is required, the final cumulative voltage of thestorage capacitors is available across the terminals 7 and 8, and isused at the desired time to fire the photo flash tube, for taking thedesired picture. The photo flash tube itself, and the control mechanismfor firing it and for quenching the flash, if desired, may be inaccordance with any conventional construction well known in the art, andso these parts are not illustratedhere in detail but are indicated onlyschematically at 100. The details of the flash tube and its controlmechanism are not important for purposes of the present invention andare subject to wide variation within the skill of the art, withoutaffecting the present invention which deals with the charging of thestorage capacitors to provide the power necessary for the flash tube,rather than dealing with the subsequent discharge or use of the storedpower in producing the flash.

What is claimed is:

1. Electronic flash apparatus comprising two storage capacitorsconnected in series with each other to store a charge to provide powerfor a flash, two resistors connected in series with each other and inparallel with the two storage capacitors in such manner as to besubjected to the working voltage of the capacitors, and a bridgeconnection connecting a central junction between the two capacitors to acentral junction between the two resistors, said bridge connectionhaving a capacitor, an impulse transformer in parallel with suchcapacitor, and a break-through device becoming conductive when subjectedto a predetermined difference in potential arranged in series with saidtransformer and in parallel with said last named capacitor, saidapparatus being so arranged that when the voltage across one of saidstorage capacitors differs by more than a predetermined amount from thevoltage across the other of said storage capacitors, said break-throughdevice will become conductive and the bridge connection capacitor willdischarge therethrough and through said transformer to activate saidtransformer.

2. Flash apparatus as defined in claim 1, wherein the resistance ratiosof said two resistors correspond respectively to the desired chargingvoltage ratios of the two storage capacitors.

3. Flash apparatus as defined in claim 1, wherein said break-throughdevice is a diac.

4. Flash apparatus as defined in claim 1, wherein said break-throughdevice is a glow lamp.

5. Flash apparatus as defined in claim 1, wherein said break-throughdevice is a four-layer diode.

6. Flash apparatus as defined in claim 1, wherein said break-throughdevice is a unijunction transistor.

7. Flash apparatus as defined in claim 1, further including meansresponsive to activation of said transformer for at least temporarilyblocking further charging of at least one of said storage capacitors.

8. Flash apparatus as defined in claim 1, further including meansresponsive to activation of said transformer for at least temporarilyblocking further charging of that one of said storage capacitors whichis charged to the higher voltage.

9. Flash apparatus as defined in claim 1, wherein said impulsetransformer has a primary winding in series with said break-throughdevice and two secondary TJNTTTD STATES PATENT oTTTsE QERTTHCATE 9FQQRREUTWN Patent No. 3,681,672 Dated August 1, 1972 lnventor (s)Kar1-PeteT Strauss It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

On the cover sheet insert:

[32] Priority July 2, 1970 33] Germany Signed and sealed this 19th dayof December 1972,

(SEAL) Attest:

EDWARD MELETGHERMR, ROBERT GOTTSCHALK Attesting Officer Commissioner ofPatents FORM PO-1050 (10-69) USCOMM-DC 60376-P89 W U.S. GOVERNMENTPRINTING OFFiCE: 1968 O36G33 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTIQN Patent No. 3,681,672 Dated August 1, 1972 lnventol-(s) Karl-Peter Strauss It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

On the cover sheet insert:

[32] Priority July 2, 1970 {33} Germany Signed and sealed this 19th dayof December 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. v ROBERT GOTTSCHALK Attesting Officer vCommissioner of Patent;

FORM PO-105O (10-69) USCOMM-DC 60376-1 69 1* us. GOVERNMENT PRINTINGorncz: i969 O366-33A.

1. Electronic flash apparatus comprising two storage capacitorsconnected in series with each other to store a charge to provide powerfor a flash, two resistors connected in series with each other and inparallel with the two storage capacitors in such manner as to besubjected to the working voltage of the capacitors, and a bridgeconnection connecting a central junction between the two capacitors to acentral junction between the two resistors, said bridge connectionhaving a capacitor, an impulse transformer in parallel with suchcapacitor, and a break-through device becoming conductive when subjectedto a predetermined difference in potential arranged in series with saidtransformer and in parallel with said last named capacitor, saidapparatus being so arranged that when the voltage across one of saidstorage capacitors differs by more than a predetermined amount from thevoltage across the other of said storage capacitors, said break-throughdevice will become conductive and the bridge connection capacitor willdischarge therethrough and through said transformer to activate saidtransformer.
 2. Flash apparatus as defined in claim 1, wherein theresistance ratios of said two resistors correspond respectively to thedesired charging voltage ratios of the two storage capacitors.
 3. Flashapparatus as defined in claim 1, wherein said break-through device is adiac.
 4. Flash apparatus as defined in claim 1, wherein saidbreak-through device is a glow lamp.
 5. Flash apparatus as defined inclaim 1, wherein said break-through device is a four-layer diode. 6.Flash apparatus as defined in claim 1, wherein said break-through deviceis a unijunction transistor.
 7. Flash apparatus as defined in claim 1,further including means responsive to activation of said transformer forat least temporarily blocking further charging of at least one of saidstorage capacitors.
 8. Flash apparatus as defined in claim 1, furtherincluding means responsive to activation of said transformer for atleast temporarily blocking further charging of that one of said storagecapacitors which is charged to the higher voltage.
 9. Flash apparatus asdefined in claim 1, wherein said impulse transformer has a primarywinding in series with said break-through device and two secondarywindings (27, 28), said two storage capacitors (1, 2) have chargingcircuits separately controlled by two trigger-controlled rectifierthyristors (3, 4), and each of said secondary windings (27, 28) isoperatively connected through a respective individual diode (29, 30) toa trigger circuit for the respective rectifier thyristor (3, 4) in suchmanner that passage of trigger impulses to the corresponding thyristor(3, 4) is blocked or short circuited.